In recent years, with the growth of the print-on-demand market, the demands for speeding up electrophotographic techniques are all the more increasing. In view of the above, as a means for meeting the requirement of speeding up, a toner that is capable of fixing at a low temperature is studied in order to fix the toner on paper with less energy. For example, a toner having excellent low-temperature fixing ability by containing a linear low-softening point polyester as a resin binder (see JP-B-3003936 (U.S. Pat. No. 5,079,123), JP-B-3415909 (U.S. Pat. No. 5,395,726), and the like); a method for producing a toner including the step of melt-kneading under specified conditions, using a resin binder containing a polyester having a softening point of from 90° to 110° C. and a low-melting point wax having a melting point of from 60° to 90° C. (see JP-A-2006-47879, and the like); and the like are proposed.
Further, a toner is scattered by speeding up, thereby making it more likely to generate the phenomenon of staining inside the machine; therefore, in order to obtain a high triboelectric chargeability, a polyester obtained from an aromatic monomer such as terephthalic acid as a raw material monomer for a resin binder is widely used. In addition, with the speeding up, although mechanical stress applied to a toner also increases, the glass transition temperature of the polyester becomes high by using the above aromatic monomer, thereby also increasing durability (see JP-A-2003-149865, and the like).
On the other hand, in order to satisfy both low-temperature fixing ability and storage stability of the toner, JP-A-2003-29460 proposes a method for producing a toner including the step of melt-kneading at a specified temperature a resin binder containing a crystalline polyester, a polyester having a softening point of from 120° to 170° C., a glass transition temperature of from 58° to 75° C., and a percentage of chloroform-insoluble components of from 5 to 50% by mass, and a polyester having a softening point of from 90° to 120° C., a glass transition temperature of from 58° to 75° C., and a percentage of chloroform-insoluble components of less than 5% by mass, and the publication discloses a toner containing a polyester of which raw material monomer is isophthalic acid, and a polyester of which raw material monomer is fumaric acid. In addition, in order to exhibit high gloss, JP-A-Hei-4-338973 proposes a toner containing a first nonlinear polyester having a softening point of 105° C. or more and less than 120° C. and a second nonlinear polyester having a softening point of 80° C. or more and less than 105° C., and the publication discloses a toner containing a polyester in which the first polyester or the second polyester is a polyester of which raw material monomers are isophthalic acid and fumaric acid.
In addition, as a toner capable of maintaining high image qualities even for a long period of use, having very small influences therein even under both low-temperature, low-humidity and high-temperature, high-humidity environmental conditions, having high transparency, fixable at low temperatures, and capable of forming a smooth fixing surface, JP-A-Hei-8-30027 proposes a toner in which a resin binder is a linear polyester, and the resin binder has a pulverizability index of from 14 to 40, and the publication discloses a toner containing a polyester of which raw material monomers are isophthalic acid and fumaric acid.
However, the isophthalic acid monomer in these toners is used in a ratio of only a small amount of less than 8% by weight of the entire raw material monomers for the polyester in the toner.